LIQUID HYDROGEN
LIQUID HYDROGEN
The most common way to store hydrogen in a liquid form is to cool it down to cryogenic temperatures (–253 °C). Other options include storing hydrogen as a constituent in other liquids, such as NaBH4
solutions, rechargeable organic liquids, or anhydrous ammonia NH 3 . This section discusses the three most promising methods: cryogenic H 2 , NaBH 4 solutions, and rechargeable organic liquids.
24 HYDROGEN PRODUCTION AND STORAGE R&D PRIORITIES AND GAPS
Cryogenic liquid hydrogen (LH 2 )
Cryogenic hydrogen, usually simply referred to as liquid hydrogen (LH 2 ), has a density of 70.8 kg/m 3 at normal boiling point (–253 °C). (Critical pressure is 13 bar and critical temperature is –240 °C.) The theoretical gravimetric density of LH 2 is 100%, but only 20 wt. % H 2 of this can be achieved in practical hydrogen systems today. On a volumetric basis, the respective values are 80 kg/m 3 and 30 kg/m 3 . This means that liquid hydrogen has a much better energy density than the pressurised gas solutions mentioned above. However, it is important to recall that about 30-40% of the energy is lost when LH 2 is produced. The other main disadvantage with LH 2 is the boil-off loss during dormancy, plus the fact that super-insulated cryogenic containers are needed. The general public’s perception of LH 2 as an unsafe and very high-tech system should not be underestimated. The main advantage with LH 2 is the high storage density that can be reached at relatively low pressures. Liquid hydrogen has been demonstrated in commercial vehicles (particularly by BMW), and in the future it could also be co-utilized as aircraft fuel, since it provides the best weight advantage of
any H 2 storage. The main R&D tasks are to:
Develop more efficient liquefaction processes (hydride compressors, magnetic and acoustic cooling, etc.).
Lower costs and improve the insulated containers.
Develop systems that automatically capture the boil-off (e.g. via hydrides) and re-liquefy the fuel.
NaBH 4 solutions
Borohydride (NaBH 4 ) solutions can be used as a liquid storage medium for hydrogen. The catalytic hydrolysis reaction is:
NaBH 4 (l) + 2H 2 O (l) ➞ 4H 2 (g) + NaBO 2 (s) (ideal reaction) Eqn. 1 The theoretical maximum hydrogen energy storage density for this reaction is 10.9 wt.% H 2 , the
ideal reaction being 4H 2 /(NaBH 4 + 2H 2 O). The specific cost (USD/kg) of hydrogen storage using NaBH 4 solutions is quite simple to calculate:
Eqn. 2 The main advantage with using NaBH 4 solutions is that it allows for safe and controllable
Cost H 2 = 4.69 × Cost NaBH 4 (ideal reaction)
onboard generation of H 2 . The main disadvantage is that the reaction product NaBO 2 must
be regenerated back to NaBH 4 off-board. Although use of NaBH 4 solutions in vehicles may
be prohibitively expensive (the cost of NaBH 4 regeneration must be reduced from present
50 USD/kg to less than 1 USD/kg), there do exist a few commercial companies that promote the technology (Millennium Cell in the U.S. and MERIT in Japan). The required cost reduction is unlikely because of the unfavourable thermodynamics. However, NaBH 4 solutions may be usable in high-value portable and stationary applications. The following R&D tasks have been identified:
Research how close one can approach the ideal energy density (10.9 wt.% H 2 ) by optimising the H 2 O needed in the reaction (Eqn. 1) and develop methods to obtain H 2 O from the fuel cell.
Develop practical NaBO 2 removal, regeneration, and replacement methods.
Develop a direct borohydride fuel cell.
HYDROGEN STORAGE R&D: PRIORITIES AND GAPS 25
Rechargeable organic liquids
Some organic liquids can also be used to indirectly store hydrogen in liquid form. The following three steps summarise the basic concept. First, an organic liquid is dehydrogenated (in a catalytic
process) to produce H 2 gas onboard. Second, the dehydrogenated product is transported from the vehicle tank to a central processing plant, while simultaneously refilling the tank with fresh
H 2 -rich liquid. Finally, the H 2 -depleted liquid needs to be re-hydrogenated, brought back to the starting compound and returned to the filling station.
One example of a rechargeable organic liquid process is the dehydrogenation and hydrogenation
of methylcyclohexane (C 7 H 14 ) and toluene (C 7 H 8 ):
C 7 H 14 (l) ⇔ C 7 H 8 (l) + 3 H 2 (g) (T dehyd = 300-400 °C) Eqn. 3 The ideal reaction in Eqn. 3 yields a gravimetric and volumetric H 2 energy storage density of
6.1 wt.% H 2 and 43 kg H 2 /m 3 , respectively. It should also be noted that the organic liquids involved in this reaction (Eqn. 3) must be handled with great care (methylcyclohexane is a clear colorless liquid that reacts violently with strong oxidants, causing fire and explosion hazards). This means that it is necessary to perform detailed safety and toxicity studies. General studies on possible infrastructure scenarios and corresponding cost calculations should also be performed. The more specific R&D tasks are to:
Develop organic systems that can be dehydrogenated at low temperatures and produce H 2 at feasible pressures.
Develop optimal metal dehydrogenation catalysts and onboard systems.
Develop the re-hydrogenation process.
A comparison of the merit factors for liquid hydrogen (LH 2 ), borohydride (NaBH 4 ) solutions and the organic liquids discussed above is presented in Table 6. In general it can be concluded that the handling and transport of liquid hydrogen, which may involve highly toxic chemical substances or
extreme temperatures, requires a safe and well-organised industrial infrastructure. The H 2 -liquid production (or regeneration) infrastructure would have to be distributed in order to minimise the transport cost to the distributed refuelling stations. The build-up of such infrastructure could be quite costly and should
be combined with non-vehicular applications like stationary power production and aviation transport. LH 2 could meet the fuel demands for aviation, while the other two options (borohydride solutions and organic liquids) could be suitable for refuelling of terrestrial transport (e.g. private cars and fleet vehicles).
Table 6 Merit factors for liquid H 2 storage: comparison of LH 2 , NaBH 4 solutions
and organic liquids
Parameter
Organic liquids Value
LH 2 NaBH4 solutions
Value Comment
Temperature, T –
T dehyd = 300-400 °C Pressure, p
30-40% losses
Low p
6.1 wt.% H 2 Safety
Energy density* +
100 wt.% H 2 †
10.9 wt.% H 2 +
Toxicity Cost
Public perception
Infrastructure * Theoretical maximum.
Infrastructure
Regeneration costs
20 wt.% H 2 in practical LH 2 systems.
26 HYDROGEN PRODUCTION AND STORAGE R&D PRIORITIES AND GAPS